Can anyone help locate the NFB circuit on this amp?

[tone?]

here is a guy shot
the Impedance switch is on the top right hand corner

http://www.flickr.com/photos/21189903@N ... otostream/


I want to locate the resistor so i can jumper it or change it to make the amp looser and a tad harier

thanks!

[mightymike]

I can't mak3 heads or tails from that pic, but I'm sure if you asked VHT which resister is the NFB that they would tell you. It's not like they covered it in goup. They might even send you a schematic.

Where's the presence pot?

[tone?]

here is the presence pot on the left and reverb on the right

[mightymike]

See that purple wire going under the board? That's prob your Nfb wire. I would look under to see that it is going to that pot. I bet it is. Unbolt the pots from the chasis if you have to. What ever long wire is going there to the imp switch or speaker jack (Usually purple) will be it. Then look for a resister between 27k up to 100k along that wire in series with it. That yellow box on the board behind the pot is prob a .68uf.

That's a 70s style Marshall presence circuit like this
http://img687.imageshack.us/img687/5231 ... d04par.jpg" onclick="window.open(this.href);return false;

Where's your speaker jack?

[tone?]

on the other side
I have the impedance switch circled in Red and next to it are the speaker jacks.

here is a high res

http://www.flickr.com/photos/21189903@N ... otostream/

[stef]

It's a bad idea to mess with this motherboard

[tone?]

It's a bad idea to mess with this motherboard

yeah I know!!!

LOL

[tone?]

Its like I learned a bit how to fix a bike and then decided I could mess with the space shuttle

LOL

[mightymike]

22k is an off value, but R1on the little board could be it. Need to unbolt the reverb and presence pot to turn them and the board over to check. I don't know why the NFB (purple wire) doesn't just go straight across to the speaker jacks or imp switch. It's a straight shot across. Must be something else going on for them to route it to the side

[mightymike]

Is there a variable negative feedback control on your amp? If there is I would imagine a 50k to 100k pot in series with that 22k. Then the wire from the pot going to the speaker jack or impedence switch which woulr explain the routing towards the control of the purple.

So I guess I should ask what all your controls say. Is there a reasonace control? That the purple wire goes to?

[tone?]

The purple wire goes to the transformer ?


A grey wire goes to the impedance switch

[tone?]

Is there a variable negative feedback control on your amp? If there is I would imagine a 50k to 100k pot in series with that 22k. Then the wire from the pot going to the speaker jack or impedence switch which woulr explain the routing towards the control of the purple.

So I guess I should ask what all your controls say. Is there a reasonace control? That the purple wire goes to?

Thanks for all your help mike !


No there is no resonance control

It has switches for class a or ab
Tube or ss recitfier

Next to the presence is the reverb , then gain , volume treble mids
Basic stuff

[mightymike]

Hmmm is there a depth control?

[tone?]

Is there a variable negative feedback control on your amp? If there is I would imagine a 50k to 100k pot in series with that 22k. Then the wire from the pot going to the speaker jack or impedence switch which woulr explain the routing towards the control of the purple.

So I guess I should ask what all your controls say. Is there a reasonace control? That the purple wire goes to?

The other resistor on that little board is 100k next to the 22k

But if you look at the other pic the purple wire goes outside the chassis towards the transformer

[tone?]

Hmmm is there a depth control?

Nope no depth